// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.

#ifndef STORAGE_LEVELDB_UTIL_RANDOM_H_
#define STORAGE_LEVELDB_UTIL_RANDOM_H_

#include <cstdint>

namespace leveldb {

// A very simple random number generator.  Not especially good at
// generating truly random bits, but good enough for our needs in this
// package.
    class Random {
    private:
        uint32_t seed_;

    public:
        explicit Random(uint32_t s) : seed_(s & 0x7fffffffu) {
            // Avoid bad seeds.
            if (seed_ == 0 || seed_ == 2147483647L) {
                seed_ = 1;
            }
        }

        uint32_t Next() {
            static const uint32_t M = 2147483647L;  // 2^31-1
            static const uint64_t A = 16807;        // bits 14, 8, 7, 5, 2, 1, 0
            // We are computing
            //       seed_ = (seed_ * A) % M,    where M = 2^31-1
            //
            // seed_ must not be zero or M, or else all subsequent computed values
            // will be zero or M respectively.  For all other values, seed_ will end
            // up cycling through every number in [1,M-1]
            uint64_t product = seed_ * A;

            // Compute (product % M) using the fact that ((x << 31) % M) == x.
            seed_ = static_cast<uint32_t>((product >> 31) + (product & M));
            // The first reduction may overflow by 1 bit, so we may need to
            // repeat.  mod == M is not possible; using > allows the faster
            // sign-bit-based test.
            if (seed_ > M) {
                seed_ -= M;
            }
            return seed_;
        }

        // Returns a uniformly distributed value in the range [0..n-1]
        // REQUIRES: n > 0
        uint32_t Uniform(int n) { return Next() % n; }

        // Randomly returns true ~"1/n" of the time, and false otherwise.
        // REQUIRES: n > 0
        bool OneIn(int n) { return (Next() % n) == 0; }

        // Skewed: pick "base" uniformly from range [0,max_log] and then
        // return "base" random bits.  The effect is to pick a number in the
        // range [0,2^max_log-1] with exponential bias towards smaller numbers.
        uint32_t Skewed(int max_log) { return Uniform(1 << Uniform(max_log + 1)); }
    };

}  // namespace leveldb

#endif  // STORAGE_LEVELDB_UTIL_RANDOM_H_
